Due to continuously more stringent emission requirements, internal combustion engines are equipped with aftertreatment exhaust gas systems, such as Diesel Oxidation Catalyst (DOC), Diesel Particulate Filters (DPF), Lean NOx Traps (LNT), and/or a Selective Catalytic Reduction (SCR) systems or SCRFs (SCR on and so on, whose conversion efficiency is strictly related to the exhaust gas temperature. Future required levels of CO2 imposed by legislation will drive towards lower temperatures in the exhaust line, generating critical working conditions that may impact the ability of the aftertreatment systems to operate at optimal efficiency.
Several valve control technologies have been developed; one of these is Variable Valve Actuation (VVA), in terms of valve timing or lift, allowing adopting optimized cam lobe profiles for intake and/or exhausting valves. Known Diesel application of VVA is mainly intended to reinforce the swirl of fresh charge, for example by actuation of one intake valve, or to generate internal Exhaust Gas Recirculation (EGR) by reopening of exhaust valves, improving the capability to meet emissions certification requirements.
In customer-related real driving conditions, aftertreatment systems need to be frequently regenerated in order to ensure the required conversion efficiency, and the regeneration processes do not have a negligible impact on fuel consumption. To be effective, a regeneration process, either De-sulphation for a LNT or Soot oxidation for a DPF or other regeneration processes, typically requires a sufficiently high and stable exhaust gas temperature upstream of the aftertreatment system.
The exhaust gas temperature is strictly related to the engine speed and load values, therefore in current internal combustion engines, the use of extra-fuel through after or post injections, namely late injections scheduled at open exhaust valves, or by a dedicated fuel injector provided in the exhaust line is deployed to ensure the needed temperature levels during aftertreatment regenerations.
Another issue may be given by oil dilution due to portions of fuel reaching the oil sump and consequent oil life deterioration when performing post-injections, with impact on the customer due to more frequent service need.
Emissions and CO2 deterioration are also to be accounted for during homologation cycles.
A further issue is given by a reduced efficiency of aftertreatment processes, such as DPF regeneration or LNT De-sulphation, due to unstable temperature under customer real driving conditions, leading to high unburned soot residuals or lower Sulphur removed at the end of the regeneration process. That means increased regeneration duration or frequency, penalizing fuel economy, due to higher number of regeneration events over component life, system performance deterioration (oxidation capacity) due to thermal stress or ageing because of increased time exposure to high temperatures reached during regeneration process, with potential drawbacks on emissions. Also, a high calibration effort of post-injection control may be needed to enable a sufficiently stable upstream aftertreatment system temperature in every driving conditions.
Finally, during real customer driving conditions, the city cycle is one of the most critical profiles for regeneration process effectiveness: the frequent operations at low engine speed and load with accelerations, decelerations and idle engine speed lead to a highly variable and unstable temperature at the engine outlet and to a consequent very complex control of the post-injection to guarantee the achievement of the required temperatures.